Atomic absorption spectrometers analyse a sample by burning the sample in a flame and passing electromagnetic radiation through the flame so that the sample atoms absorb particular wavelengths of the radiation. By detecting the radiation which is absorbed by the ground state atoms in the flame the conception of the elements of interest can be determined.
In order to analyse the sample the atomic absorption spectrometer includes a burner assembly which has a spray chamber which is fluid communication with a burner. The spray chamber has a nebuliser bung which receives sample material from an inlet tube. The sample material is generally entrained in a flow of fluid. The nebuliser bung includes a baffle which has an orifice and the tube terminates adjacent the inlet to the orifice. An oxidant line communicates with the nebuliser bung on the inlet side of the orifice and a fuel and air line communicates with the nebuliser bung on the outlet side of the orifice. The bung is in fluid communication with the spray chamber so that when high pressure oxidant passes through the oxidant line, the high pressure oxidant travels through the orifice thereby creating a venturi effect within the sample tube which assists in the drawing of sample through the sample tube into the nebuliser bung and then into the spray chamber. Fuel and air supplied through the fuel line into the nebuliser bung mixes with the oxidant and sample in the spray chamber and the mixture is supplied to the burner where the fuel, air and sample are combusted in a flame for analysis.
The spray chamber has an over pressure relief device in the form of an over pressure bung which locates in an aperture in the spray chamber so that in the event of over pressure in the spray chamber the bung is forced out of the opening to relieve that pressure. An over pressure situation can occur if pressure initially drops within the spray chamber so that the flame produced by the burner can move inside the spray chamber and causes flashback. If this occurs the increase in the pressure caused by the flashback will cause the bung to be expelled from the opening thereby reducing the pressure and reducing the damage caused by the flashback.
If the bung is not located within the opening, fuel gas can leak from the spray chamber into the environment which, if the burner was to be ignited, can create an explosion.
Thus, if the spray chamber assembly components are not inserted correctly, the highly flammable gas produced by the mixture of the fuel and air may be allowed to leak into atmosphere. Such a leak can have two hazardous outcomes;                the velocity of the gasses exiting burner is reduced to well below the flame velocity causing the flash-back referred to above; and        sufficient fuel gas leaks into the environment to create an explosive atmosphere if, for example, a bung is not located in the opening or incorrectly located in the opening.        
Similar hazards can also arise from using the wrong burner in the assembly, or allowing the burner to become excessively clogged (from salt formations etc). For these reasons, most gas burner assembly designs rely on interlocks to guard the operator from such hazards. These interlocks are typically realised by means of micro-switches or read switches which sense the presence of components in the assembly. A typical example is a micro-switch which is activated by the presence of the over pressure relief device inserted into the spray chamber body. If the micro-switch is not activated by proper location of the over pressure relief device or bung, the instrument will not ignite the flame or vent fuel into the environment.
Burner assemblies are also provided with a liquid trap for draining off aspirated liquid which is supplied to the chamber with the sample. The liquid trap is typically in the form a u-shaped tube into which liquid can drain. The u-shaped tube forms an “s-bend” so that liquid in the s-bend acts as a plug to prevent leakage of flammable gas from the spray chamber through the liquid trap. As liquid flows into the liquid trap, the liquid can flow out of the s-bend but a sufficient amount of liquid remains in the s-bend to form a plug thereby preventing the escape of gasses from the spray chamber through the liquid trap. A magnet may be provided with floats on the surface of the liquid and when the liquid level is at a height sufficient for safe operation (indicating that the liquid plug is in place), the magnet triggers a reed switch to provide a signal indicative of the fact that the integrity of the liquid trap is in tact.